Recently, various methods of transmission channel allocation have been proposed to realize effective use of frequency, in mobile communication systems such as mobile telephones, of which some have been practically implemented.
FIG. 22 shows an arrangement of channels in various communication systems including FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and PDMA. Referring to FIG. 22, FDMA, TDMA and PDMA will be briefly described.
FIG. 22(a) represents channel arrangement of FDMA, in which analog signals of users 1 to 4 are subjected to frequency division and transmitted over radio waves of different frequencies f1 to f4, and the signals of respective users 1 to 4 are separated by frequency filters.
FIG. 22(b) represents a channel arrangement of TDMA, in which digitized signals of respective users are transmitted over the radio waves having different frequencies f1 to f4 and time-divided time slot by time slot (time slot: a prescribed time period), and the signals of respective users 1 to 8 are separated by the frequency filters and time-synchronization between a base station and mobile terminals of respective users.
Recently, PDMA method has been proposed to improve efficiency of use of radio frequency, as mobile telephones have come to be widely used. In the PDMA method, one time slot of one frequency is spatially divided to enable transmission of data of a plurality of users, as shown in FIG. 22(c). In the PDMA, signals of respective users are separated by the frequency filters, the time synchronization between the base station and the mobile terminals of respective users, and a signal extracting apparatus such as an adaptive array.
FIG. 23 represents a reception system of a conventional PDMA base station. In this example, in order to identify users 1 and 2, four antennas 3 to 6 are provided, outputs of respective antennas are applied to a frequency converting circuit 7, subjected to frequency conversion by corresponding local oscillation signal Lo, respectively, converted to digital signals by an A/D converter 8 and applied to a digital signal processor (DSP) 10.
DSP 10 includes adaptive arrays 11, 12, a reception signal vector calculator 13, a memory 14, a correlation value calculator 15 and a channel allocating apparatus 16. Adaptive arrays 11 and 12 extract, from reception signals output from A/D converter 8, only those signals from a specific user. Each adaptive array extracts a user signal designated by channel allocating apparatus 16, which will be described later, in accordance with a weight vector calculating method such as a method utilizing a preamble included in a time slot or a method utilizing a nature that an envelop of a modulation signal becomes constant, for example.
Reception signal vector calculator 13 receives as inputs the reception signals from A/D converter 8 and output signals from adaptive arrays 11, 12, calculates reception signal vectors corresponding to every user, and stores the results in memory 14. Channel allocating apparatus 16 designates two users for the memory 14 and the correlation value calculator 15. Correlation value calculator 15 calculates, among the reception signal vectors stored in memory 14, correlation value between reception signal vectors of the designated two users. Channel allocating apparatus 16 receives the calculated correlation value between the reception signal vectors of the two users. When the correlation value is not larger than a prescribed value, the two users are subjected to path division multiple connection to a time slot of the same time.
Adaptive arrays 11 and 12 shown in FIG. 23 extract signals of corresponding users 1 and 2, respectively. When a user 3 transmits a signal from the same direction as user 1, in addition to users 1 and 2, it follows that the signals from users 1 and 3 are mixed and output from adaptive array 11. The conventional adaptive array 11, however, cannot separate the signals of users 1 and 3, and hence, it has been impossible to extract the signal of user 1 only.
Therefore, an object of the present invention is to provide a radio reception system that can improve communication quality, by canceling unnecessary user signals using an interference canceller.